The present invention is directed, in general, to transient voltage surge suppression apparatus and, more specifically, to improved modular designs for such apparatus.
For many years, manufacturers of electronic systems have recommended that users take measures to isolate their hardware from transient overvoltages (also called xe2x80x9csurgesxe2x80x9d) that may cause damage to sensitive electronic devices. Transient voltage protection systems (so-called xe2x80x9csurge suppressorsxe2x80x9d) are designed to reduce transient voltages to levels below hardware-damage susceptibility thresholds; providing such protection can be achieved through the use of various types of transient-suppressing elements coupled between the phase, neutral and/or ground conductors of an electrical distribution system.
Conventional transient-suppressing elements typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a pre-determined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and xe2x80x9cshuntingxe2x80x9d the current associated with the transient voltage through the element and thus away from the sensitive electronic hardware to be protected.
To be reliable, a transient-suppressing element itself must be capable of handling many typical transient-voltage disturbances without internal degradation. This requirement dictates the use of heavy-duty components designed for the particular transient voltage environment in which such elements are to be used. In environments characterized by high-magnitude or frequently-occurring transients, however, multiple transient-suppressing elements may be required.
In many applications, the transient-suppressing elements typically employed are metal-oxide varistors (xe2x80x9cMOVsxe2x80x9d); silicon avalanche diodes (SADs) and gas tubes are other types of transient-suppressing elements. When designing a system incorporating MOVs it is important to recognize the limitations of such devices, and the effects that the failure of any given MOV may have on the integrity of the total system. All MOV components have a maximum transient current rating; if the rating is exceeded, the MOV may fail. An MOV component may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by an MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure. A designer of transient voltage protection systems must consider these electrical environment factors when selecting the number and type of MOVs to be used in a particular system. Therefore, to design a reliable transient voltage suppression system, a designer must consider both the maximum single-pulse transient current to which the system may be subjected, as well as the possible frequency of transients having lower-level current characteristics.
Although individual MOVs have a maximum transient current rating, it is possible to construct a device using multiple MOVs, in parallel combination, such that the MOVs share the total transient current. In this manner, each individual MOV must only conduct a fraction of the total transient current, thereby reducing the probability that any individual MOV will exceed its rated maximum transient current capacity. Furthermore, by using a plurality of individual MOVs, a transient voltage protection system can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV.
When a transient voltage suppression system incorporates multiple MOVS, it is important that the system be designed such that the failure of an individual MOV does not cause a complete loss of system functionality. When an MOV fails, due to either exceeding its maximum transient current rating or frequent operation, it initially falls into a low impedance state, drawing a large steady-state current from the electrical distribution system. This current, if not interrupted, will quickly drive an MOV into thermal runaway, typically resulting in an explosive failure of the MOV.
To avoid the explosive failure of MOVs, an appropriately-rated current-limiting element, such as a fuse, should be employed in series with MOVs. If the transient-suppressing device incorporates a plurality of parallel-coupled MOVs, however, a single fuse in series with the parallel combination of MOVs may open-circuit even if only a single MOV fails, resulting in a disconnection of the remaining functional MOVs from the electrical distribution system. Therefore, better-designed systems incorporate individual fuses for each MOV, such that the failure of an individual MOV will result only in the opening of the fuse coupled in series with the failed MOV; the remaining functional MOVs remain connected to the electrical distribution system, via their own fuses, to provide continued transient voltage protection.
In the prior art, there are transient suppression circuits that incorporate a plurality of parallel-coupled MOVs with an individual fuse provided for overcurrent protection of the MOVs. U.S. Pat. No. 5,153,806 to Corey teaches the use of a single fuse to protect a plurality of MOVs, as well as an alarm circuit for indicating when the fuse has open-circuited. Similarly, U.S. Pat. No. 4,271,466 to Comstock teaches the use of a single fuse in series with a plurality of MOVs, as well as a light-emitting diode (xe2x80x9cLEDxe2x80x9d), coupled in parallel with the fuse, to emit light when the fuse is blown. The deficiencies of these types of circuits is that the failure of a single MOV can cause the fuse to fail whereby the remaining functional MOVs are decoupled from the circuit; i.e., the remaining functional MOVs are disconnected from the electrical distribution system and thus cannot provide continued protection from transient voltages.
There are also a limited number of transient suppression devices that employ multiple over-current limiting elements with multiple parallel-coupled MOVs or other transient suppression devices. Such devices known in the prior art, however, typically employ a bare fusible element mounted on the printed circuit board on which the MOVs are mounted. When an MOV associated with a particular fusible element fails, the fusible element typically open circuits. The open-circuiting of a fusible element is often accompanied by electrical arcing, which is particularly true in the area of transient suppression devices because of the large voltages and currents usually present when a suppression device fails. Because of the close proximity of the bare fusible elements, the electrical arcing of one fusible element can result in the destruction of adjacent elements, thereby decoupling remaining functional MOVs from the circuit and further limiting the remaining suppression capacity of the device.
The inadequacy of the prior art is that the failure of a single MOV component may cause a current-limiting element, such as a fuse, in series with a plurality of parallel-coupled MOVs to open-circuit, thus eliminating all transient voltage suppression capability of the parallel-coupled MOVs. In prior art circuits that have employed multiple current-limiting elements with multiple parallel-coupled MOVs (or other transient suppression devices), the failure of a current-limiting element can cause electrical arcing that can result in the destruction of adjacent current-limiting elements, or MOVs, thus resulting in further degradation of the suppression capacity of the circuit. Therefore, there is a need in the art for improved apparatus for providing over-current protection to a plurality of parallel-coupled transient-suppression devices; such improved apparatus preferably reduce, or eliminate, the possibility of failures due to electrical-arcing.
As described supra, it is known in the prior art to provide multiple MOVs, in parallel combination, such that the MOVs share the total transient current. Furthermore, such circuits can be housed in individual modules, and multiple modules can be coupled in parallel to increase the surge capacity of the device. Examples of prior art modular devices are disclosed by Ryan, et al. in U.S. Pat. Nos. 5,701,227, 5,953,193, 5,966,282, and 5,969,932, incorporated herein by reference. A particular inadequacy of such prior art modular devices, however, is the manner in which the modules are coupled together, which requires each module in a stack of modules to be independently coupled to each adjacent module. This manner of assembly increases not only the number of physical parts, but also the assembly time, as well as the disassembly time required to repair or replace a failed module. Accordingly, there is a further need in the art for improved modular structures for housing transient voltage suppression circuits.
To address certain above-described deficiencies of the prior art, the present invention provides improved modular transient voltage surge suppressor apparatus that equalize transient current sharing between multiple modules. In general, such apparatus includes first and second transient voltage surge suppression modules, each module having a non-conductive housing with a surge suppression circuit contained therein, and first and second electrically-conductive buses mechanically coupled to the non-conductive housing and electrically coupled to first and second terminals of the surge suppression circuit, respectively. A first bus coupler couples the first electrically-conductive buses of the first and second transient voltage surge suppression modules and a second bus coupler couples the second electrically-conductive buses of the first and second transient voltage surge suppression modules, whereby the surge suppression circuits in each of the first and second modules are electrically coupled in parallel. A first electrical conductor coupler is electrically coupled to, and physically located proximate, the first electrically-conductive bus of the first transient voltage surge suppression module, and a second electrical conductor coupler is electrically coupled to, and physically located proximate, the second electrically-conductive bus of the second transient voltage surge suppression module, whereby the electrical path length from the first electrical conductor coupler to the second electrical conductor coupler and through the surge suppression circuit of the first transient voltage surge suppression module is substantially equal to the electrical path length from the first electrical conductor coupler to the second electrical conductor coupler and through the surge suppression circuit of the second transient voltage surge suppression module.
In a specific embodiment illustrated and described hereinafter, such apparatus includes a substrate, with first and second mounting posts coupled to and extending substantially perpendicular thereto. First and second transient voltage surge suppression modules mounted on the mounting posts each include a non-conductive housing having a surge suppression circuit contained therein, and first and second electrically-conductive buses mechanically coupled to the non-conductive housing and electrically coupled to first and second terminals of the surge suppression circuit, respectively. The first and second electrically-conductive buses include a bore therethrough for slidably mounting the transient voltage surge suppression modules on the first and second mounting posts; the bores have internal profiles corresponding to the external profiles of the mounting posts. The first transient voltage surge suppression module is mounted on the first and second mounting posts adjacent to the substrate and the second transient voltage surge suppression module is mounted on the first and second mounting posts adjacent to the first transient voltage surge suppression module, whereby the surge suppression circuits in each of the first and second modules are electrically coupled in parallel. A first electrical conductor coupler is electrically coupled to, and physically located proximate, the first electrically-conductive bus of the first transient voltage surge suppression module, and a second electrical conductor coupler is electrically coupled to, and physically located proximate, the second electrically-conductive bus of the second transient voltage surge suppression module, whereby the electrical path length from the first electrical conductor coupler to the second electrical conductor coupler and through the surge suppression circuit of the first transient voltage surge suppression module is substantially equal to the electrical path length from the first electrical conductor coupler to the second electrical conductor coupler and through the surge suppression circuit of the second transient voltage surge suppression module.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject matter of the claims recited hereinafter. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.